This invention relates to a laser beam optical focusing system and, more particularly, to an optical focusing system with two symmetrical diffractive optical elements.
Laser beams have wide-ranging applications such as heat annealing or vaporization, optical memory recording and playback, optical scanning and xerographic and printing use. For these diverse applications, a laser beam should be a focused beam. Various optical systems have been proposed over the years to provide focusing means for a laser beam.
Optical focusing systems in modern day apparatus are becoming more accurate on the one hand but more complicated and expensive on the other hand.
A compact design for the focusing optics of any optical system is always desirable to make the entire optical system itself as compact as possible and to enable extension of the same design into many architectures.
It would be desirable to improve the efficiency, shorten optical path lengths and use as few optical elements as possible to decrease hardware, assembly and alignment costs of these types of optical focusing systems.
A semiconductor laser or laser diode emits a diverging light beam. The prior art uses optical elements, typically lenses but sometimes mirrors, to shape and focus the angular diverging beam emitted from a semiconductor laser to a focused spot size.
A single field point in a prior art focusing system with refractive lenses can easily be optimized given enough degrees of freedom in the lenses. However, with a large field of points and a small spot size, the design of an optical focusing refractive lens system is difficult.
Prior art focusing systems typically involve several refractive lens elements configured in an array of lenses. All the lens elements would have to be virtually identical. Fabrication of identical lens elements presents a problem when tolerances have to be on the order of 1 micron or better for refractive elements. Fabrication of lenses by molding involves heat. Heat can cause differences in thickness and refractive index from fabricated lens to fabricated lens, even if the lens are part of the same batch under identical manufacturing circumstances. Also, axial alignment of lens within a focusing system is close to impossible.
The propagation of a light beam can be changed by three basic means: reflection by a mirror, refraction by a lens and diffraction by a grating. Optical systems traditionally rely on reflection and refraction to achieve the desired optical transformation. Optical design, based on mirror and lens elements, is a well-established and refined process. Until recently, the problems with diffraction and fabricating high efficiency diffractive elements have made diffractive elements unfeasible components of optical systems.
The diffractive process does not simply redirect a light beam. Diffraction, unlike refraction and reflection, splits a light beam into many beams--each of which is redirected at a different angle or order. The percentage of the incident light redirected by the desired angle into some given diffraction order is referred to as the diffraction efficiency for that order. The diffraction efficiency of a diffractive element is determined by the element's surface profile. If the light that is not redirected by the desired angle is substantial, the result will be an intolerable amount of scatter in the image or output plane of the optical system.
Theoretically, on-axis diffractive phase elements consisting of a grating having a given period can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase profile within any given period is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. By contrast, refractive elements are relatively wavelength insensitive. The technology for producing high quality, high efficiency, continuous phase profiles of the diffraction grating does not presently exist.
A compromise that results in a relatively high diffraction efficiency and ease of fabrication is a multi-level phase grating. The larger the number of discrete phase levels, the better the approximation of the continuous phase function. These multi-level phase profiles can be fabricated using standard semiconductor integrated circuit fabrication techniques.
As disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G. J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, Aug. 14, 1989) herewithin incorporated by reference and the resulting U.S. Pat. No. 4,895,790 also herewithin incorporated by reference, a fabrication process starts with a mathematical phase description of a diffractive phase profile and results in a fabricated multi-level diffractive surface. The first step is to take the mathematical phase expression and generate from it a set of masks that contain the phase profile information. The second step is to transfer the phase profile information from the masks into the surface of the element specified by the lens design.
The first step involved in fabricating the multi-level element is to mathematically describe the ideal diffractive phase profile that is to be approximated in a multi-level fashion. The next step in the fabrication process is to create a set of lithographic masks which are produced by standard pattern generators used in the integrated circuit industry.
A substrate of the desired material, such as Ge, ZnSe, Si, and SiO.sub.2, is coated with a thin layer of photoresist. A first lithographic mask is then placed in intimate contact with the substrate and illuminated from above with an ultraviolet exposure lamp. Alternately, pattern generators, either optical or electron beam, can expose the thin layer of photoresist. The photoresist is developed, washing away the exposed resist and leaving the binary grating pattern in the remaining photoresist. This photoresist will act as an etch stop.
The most reliable and accurate way to etch many optical materials is to use reactive ion etching. The process of reactive ion etching anisotropically etches material at very repeatable rates. The desired etch depth can be obtained very accurately. The anisotropic nature of the process assures a vertical etch, resulting in a true binary surface relief profile. Once the substrate has been reactively ion etched to the desired depth, the remaining photoresist is stripped away, leaving a binary surface relief phase grating.
The process may be repeated using a second lithographic mask having half the period of the first mask. The binary phase element is recoated with photoresist and exposed using the second lithographic mask which has half the period of the first mask. After developing and washing away the exposed photoresist, the substrate is reactively ion etched to a depth half that of the first etch. Removal of the remaining photoresist results in a 4 level approximation to the desired profile. The process may be repeated a third and fourth time with lithographic masks having periods of one-quarter and one-eighth that of the first mask, and etching the substrates to depths of one-quarter and one-eighth that of the first etch. The successive etches result in elements having 8 and 16 phase levels. More masks than four might be used, however, fabrication errors tend to predominate as more masks are used.
This process is repeated to produce a multilevel surface relief phase grating structure in the substrate. The result is a discrete, computer-generated structure approximating the original idealized diffractive surface. For each additional mask used in the fabrication process, the number of discrete phase levels is doubled, hence the name "binary" optical element or, more precisely, a binary diffractive optical element.
After only four processing iterations, a 16 phase level approximation to the continuous case can be obtained. The process can be carried out in parallel, producing many elements simultaneously, in a cost-effective manner.
A 16 phase level structure achieves 99 percent diffraction efficiency. The residual 1 percent of the light is diffracted into higher orders and manifests itself as scatter. In many optical systems, this is a tolerable amount of scatter. The fabrication of the 16 phase level structure is relatively efficient due to the fact that only four processing iterations are required to produce the element.
After the first etching step, the second and subsequent lithographic masks have to be accurately aligned to the existing pattern on the substrate. Alignment is accomplished using another tool standard to the integrated circuit industry, a mask aligner.
As noted, the photoresist on the substrate can be exposed with an electron-beam pattern generator. The e-beam direct-write process eliminates masks and their corresponding alignment and exposure problems. Binary optics have also been reproduced using epoxy casting, solgel casting, embossing, injection molding and holographic reproduction.
Binary optical elements have a number of advantages over conventional optics. Because they are computer-generated, these elements can perform more generalized wavefront shaping than conventional lenses or mirrors. Elements need only be mathematically defined: no reference surface is necessary. Therefore, diffractive optical elements can be made wavelength-sensitive for special laser systems.
The diffractive optical elements are generally thinner, lighter and can correct for many types of aberrations and distortions. It is possible to approximate a continuous phase profile with a stepwise profile of discrete phase levels.
It is an object of this invention to provide an optical focusing system with diffractive optical elements.
It is another object of this invention to provide an optical focusing system which is compact and contains inexpensive, easy to manufacture and easy to assemble optical elements.